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How accurate is CFD software compared to wind tunnel tests?
How accurate is computational fluid dynamics (CFD) software compared to wind tunnel tests?
(I mean CFD computed with super-computers like the aircraft industry has.)

Will this answer your question? Please point out where doubts remain.

yes ,these answers help..

@PeterKämpf,but in general wind tunnel test gives more accurate results compare to CFD?

Not necessarily. As I say in that other answer, aeroelasticity is impossible to match in the tunnel (or you need a new model for every data point). Also, the correction factors have only been calculated from earlier models and you only hope they match your test, too. The only accurate data can be achieved in flight test.

@Peterkampf,if you put aeroelastical wing in tunnel ,devices for lift and drag will show correct results for that tested wing.isnt it?

Yes, for that tested wing. Which is not the real wing. Good luck measuring the local bending and torsion accurately and changing the results so they fit the bending and torsion of the real wing.

@PeterKämpf,if we put full size airplane in wind tunnel and make correction for walls ,will reslults will be 100% correct as in real flight?

Yes, for 100% corrections. When this was done, the aircraft had to be held in place by steel cables which also cause differences to real flight. But full-size tests have always helped a lot, especially for drag reduction.


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Could I hold both an EASA license and a UK CAA license?
With all the ongoing mess with Brexit at the moment, I'm trying to figure out the wood from the trees so to speak. However there seems to me a bit of mass confusion with it all.
I'm just wondering if I could hold two licences at the same time? If the CAA go ahead and leave EASA, could I hold an EASA ATPL (CPL ME IR)and CAA ATPL (CPL ME IR) together?
If I can, I suspect this is in ICAO somewhere within licensing? And how would I go about it? Can I apply to on authority based on my results with the other (e.g. CAA with EASA results) or would I have to duplicate and thus spend more money to get a CAA one?

There is no regulation that says you can’t hold licenses from different authorities at the same time. Thousands of student pilots every year travel to the US, get an FAA license, then go home and get another license there.

I hold several country's pilot licenses (all ATP), although I only keep the one that I currently fly with valid, otherwise it would cost an arm and a leg, and a lot of traveling.

Welcome to aviation.SE! I don't know much about EASA and it's possible there is a good answer to this, however the effects of Brexit are still very unclear in many areas.

Yes, you can hold both. The UK CAA is now completely separate from EASA. Comparable to EASA vs FAA.

At this time, there are no agreements or associated Annexes in place between the EU and the UK in this area. Licensing and Training under the oversight of the UK CAA is now considered as third country licensing and training
Irish Aviation Authority - Brexit Notice

From EASA to a UK application: possible.
From UK to an EASA application: not possible. (Sadly EASA have "shut the door", it is very unfortunate)

If you hold an EASA licence you can apply to the UK to hold an equivalent UK Part-FCL licence. It involves the inevitable administration costs from both the UK CAA and the EASA state to send details. You will also require a UK CAA Class 1 medical - initial/revalidation at the discretion of the UK CAA AME (not the UK CAA itself).
If you are a former UK Licence holder, there are some further considerations.

UK CAA - Eligibility for application
Licence holders who fit the below criteria can apply for a UK PART-FCL licence > using this application route:
pilots, instructors and examiners who previously held a UK issued Part-FCL/BFCL/SFCL and have not had a licence issued at a higher level since 1 January 2021
EASA licence holders whose licence date of initial licence issue is prior to 31 December 2020
Licence holders who fall into the below criteria cannot use this route and must instead email FCLweb@caa.co.uk (subject heading ‘Attention Tech: EU Conversion’) for further advice:
pilots, instructors and examiners who previously held a licence with the UK CAA and have had an EASA licence issued at a higher level since 1 January 2021
EASA licence holders whose licence was issued after 31 December 2020
UK CAA - Conversion of an EASA flight crew licence to a UK part equivalent licence

Well, as it has turned out, UK need EU more than the other way around, so... but true, it is sad that political bravado messed up the affairs of a whole nation.

I would be careful to jump to conclusions here. The UK doesn’t really need pilots at the moment anyway as the largest carriers by PAX numbers (predominantly short haul due to COVID) already operate on European AOCs. By 2024, when there might actually be a serious global pilot shortage, I wouldn’t be surprised if an UK to EASA conversion process comes available. My integrated ATPL flight school had around 15-20% French students at any one time and probably another 10-20% of other EU nationals. The UK has an awful lot to offer still.

I was referring to the situation on a more general level, but you are correct on all accounts. I just wonder how on earth a nation allowed itself to get self-shafted in this way. It's almost like a Monty Python sketch...

You may need to wait until the dust settles to see exactly what the regulations end up being but if the rest of the world is an indicator then yes you can hold licenses from various places at once. Licenses can in some cases be issued simply by applying and proving your currently issued one is valid (from somewhere) as is the case with the FAA but this will likely vary by jurisdiction.

source: Stack Exchange

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sort: votes


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Can a company without an AOC hire out pilots to another company that does have one?
Is it legal for a company that does not hold an Air Operator Certificate (AOC) (company A) to hire out a pilot a second company that does have an AOC (company B)? Company B would be the one actually carrying out the aerial operations.
Would this be considered a franchise arrangement?
Edit: Company B knows about a pilot who is a sole director/owner of company A (no other employees). Company B came to company A to have the pilot fly the plane for company B's operations/contract with a third party (their pilot was unavailable).

I think it would depend on company A's stake in the deal.  A commercial pilot working for company A has a legal right to also fly for company B, and be paid for his time.  So where does company A even fit in?  If they are the ones approaching company B to give them business, how are they different than any other charter customer?  Seems like more of a legal question than aviation, but a better description of the scenario might yield a good answer here.

Company B knows about a pilot who is a sole director/owner of company A (no other employees). Company B came to company A to have the pilot fly the plane for company B's operations/contract with a third party (their pilot was unavailable).

The simple answer to your question is "NO".
There are 4 different kinds of part 135 operating certificate, many of which have a limited number of pilot positions. These pilots must be listed on the paperwork for the certificate and must meet all training, experience and currency requirements as specified by the certificate.
On a Single Pilot Operator certificate, the holder is limited to using only one pilot for all part 135 operations. That specific pilot is listed by name and certificate number on the FAA issued Operations Specification (OpSpec) A040. The use of any pilot(s) other than the single pilot listed on OpSpec A040 is not authorized.
On a Single Pilot in Command certificate, the operator is limited to using only one PIC and up to a maximum of 3 Second In Command (SIC) pilots for all part 135 operations. The PIC and the SIC(s) are listed by name and certificate number on the FAA issued OpSpec A039. The certificate holder is only authorized to use those pilots in the specific duty positions listed in OpSpec A039. The certificate holder is not authorized to use any other pilots, nor are any pilots allowed to serve in a duty position (PIC or SIC), unless they are listed in that duty position in OpSpec A039.
On a 135 Basic, the operator is limited to a maximum of 5 pilots which includes SICs, but all 5 pilots may serve as PIC or SIC as required as long as they are all so qualified. However, they must still be listed ok the certificate as above and since this type of certificate requires the establishment of approved manuals and training programs, the pilots must also have completed the necessary training and comply with the standards set forth in all operation manuals.
On a 135 Standard, there is no pre-set limit to the size of the operation, but all pilots must be listed on the certificate as above and comply with all required training and manuals as above.
Basically, an operator can't just bring in a pilot to fill-in for a single mission. If their own pilots or aircraft are not available for the job, then their company is not available for the job. They would need to sub-out or recommend the client to another certificated operator whose plane and pilot(s) are available.

This question is about Australia, so I'm not sure that part 135 is relevant.

@Pondlife: I completely overlooked the tag for Australia... Editing now.

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Is there a formula to convert SFC to air-fuel equivalence ratio λ?
Speaking about aviation internal combustion engines, is there a way to get the current value of air–fuel equivalence ratio (λ) given current specific fuel consumption (SFC) and current power?
Is the relationship between SFC and λ fixed or does it vary with engine displacement and/or power?
The data I currently have is:

The engine SFC at best power as watts/Kg*sec
The current engine power output in watts
The current engine fuel flow in Kg/sec
I know that λ is about 0.85 when SFC ratio is about 1 (best power)

No, there is no formula to calculate the air-fuel equivalence ratio from specific fuel consumption (SFC). To calculate the equivalence ratio, you need the air mass flow, and the fuel mass flow. From this data, you can calculate the actual AFR (air fuel ratio), and lookup the stoichiometric AFR for your given fuel.
$AFR = \frac{m_{air}}{m_{fuel}}$
The air-fuel equivalence ratio is then simply calculated from these two AFR values.
Air-fuel equivalnce ratio: $\lambda = \frac{AFR}{AFR_{stoich}}$
You don't have a value for the air mass flow. You only have fuel flow from the SFC and current power.
If you had the equivalence ratio and SFC at two different operating points, not just one, you could assume linear interpolation, or extrapolation, to the SFC in question.
With the equivalence ratio and SFC at just one point, you would have to find a characteristic curve for SFC vs equivalence ratio for a similar type of piston engine and the same fuel type, and assume the shape of this curve is applicable to your particular engine.

What if I know the engine displacement, compression ratio and the air density? I could get the air mass that way?

@SeeEn. You would also need the rpm to get a mass flow from displacement and rpm. This probably assumes 100% replacement of the air  every stroke. So it would be an approximation, not sure how rough.

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How do I track my airplane?
I am a student of a flying school which has 10 Cessna-152's. Since all the aircraft are not equipped with a transponder, there is no way to track them. I am searching for a way to track them like Flightradar24 does with the maps. This is a project assigned for a bunch of students, so we are looking for an inexpensive but still good quality system.

Do you need to track them in real time or just be able to come up with a track afterwards?

We need to track them real time. Thanks.

In what country?

it is for Sri Lanka

Do you have GSM signal in the flight area?

Get a transponder. (I am serious)

Define "inexpensive"...

@mins They have found aviation.SE ;-) I know what you're getting at, but if you have a school project then asking more experienced or knowledgeable people for their suggestions seems like completely reasonable research to me. In principle there's no difference between asking us and asking a local EAA chapter (or whatever).

Okay @Pondlife, I suggest requirements/accuracy/constraints are clarified.

I'm not sure if CloudAhoy is real-time or not, but with some restrictions, they are impressive.

You could try to get some cheap Car GPS trackers on aliExpress. But they would need to be in range of cell towers. Flying to high will cause connection loss.
A more expensive one, which I found to be working great is this one.
But you can get android based trackers for about 10\$US.

Would a GPS chip and Arduino work? You wouldn't need cell towers, and the Arduino could save coordinates and time (NMEA sentences) every X minutes/seconds (as desired) to an SD card or something. There's Arduino libraries/sketches that help with this, and the saved CSV could be fed into Google Earth/Maps to get a playback.

@SnakeDoc He specified real time tracking

It may be possible to use almost any Android mobile phone with GPS tracker app, somewhat like this (many others exist). Unfortunately the solution with mobile phones alone only works within the reach of cell towers, while otherwise it is both very simple and convenient.
If you have radio stations on these planes, it is possible to develop custom software stack so that mobile phone could communicate the coordinates via radio channel in some digital form, and the receiving side (another phone) should be able to decode. The audio jack on the phone has both audio output and microphone input, and data from there can make into software layer for digital processing. Converting between sound and digital data has long history and known approaches; early computers used ordinary cassete recorders to store data and software.
You need rather good software engineers in the team to realize this idea.

ITYM early hobbyist (aka home) computers like Apple, RadioShack, Commodore etc in the late 1970s. The first computers in the modern meaning (electronic, programmable) like ENIAC, EDVAC, EDSAC in the 1940s didn't use cassettes not least because they didn't exist yet. 1950s+ mainframes did use essentially the same iron-oxide-on-plastic magnetic tape as audio, but with radically different drives and circuits. Another (perhaps better) example of data over audio is the long history of modems to transmit data over telephone circults, from Bell 103 onward; you might even adapt existing devices.

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How are underground fuel pipes monitored for leakage?
This question is about environmental risk associated with high pressure lines underground at airports.
My understanding is that there is a matrix of high pressure fuel lines underground to each gate, while the source of the fuel are the main tanks. What environmental (and operational) safeguards are in-place to detect leaks to the environment?

One could make the assumption that, since this was asked on [Aviation.se] that the intent is to ask about built-in fueling systems at airports, and I'd think that would be on topic. However, some clarification from the OP would go a long way toward helping with that understanding.

Related (not answering this question): How do large airport refueling systems work?

What goes in must come out, if something is going in while nothing comes out then you have a leak.

Thank you.  The gist is environmental risk associated with high pressure lines underground.  Understanding is that there is a matrix of high pressure fuel lines underground to each gate, source of fuel is main tanks.  What environmental (and operational) safeguards are in-place to detect leaks to the environment?  Thanks again--

I have edited the question for you, next time please use the edit button instead of the comments, this will help get your question reviewed for reopening

I used to perform aerial patrol of buried petroleum product pipelines, including certain interstate transport lines. However, this question seems to be asking about the non-interstate, local transportation lines within the airport facility. I am not sure that subject is on-topic here.

@RalphJ It's aviation fuel coming out of those pipes.

@Koyovis Yes, but that still doesn't make the question about aviation, any more than a question about how dress shirts are sewn suddenly decomes on topic when we specify that the question is about how pilots' dress shirts are sewn. That question is about sewing, not flying, and this question is about the design & safeguards in fuel systems, not flying. That planes, rather than ships or racecars or whoever else needs complex underground fuel distribution, will burn the fuel doesn't change the question enough to make it on topic here. IMHO.

@RalphJ The logistics of what happens at an airport, with all the support activities such as guiding into the gate, unloading, fuelling etc are aviation related, just as loading trucks is trucking related. If pilot dress shirts had specific distinctive requirements because of their work environment, that would make the question aviation related. Is a discussion about the aluminium used in aircraft aviation related or for a metallurgic forum? There is a large gray area where topics of interest may be found, IMHO the broader the better.


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Temperature of airplane while flying?
When the air-plane reaches to the altitude of about 12,000 m or so, the airplane's outside cover is very cold due to the air temperature or very hot due to the friction? Or it depends on different parts of the airplane?

Related: https://aviation.stackexchange.com/questions/23747/what-is-the-typical-temperature-of-an-airliners-hull-during-flight


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How can I calculate the thrust of a fan given the diameter of the blade?
How do I calculate the thrust of a multicopter fan blade from its diameter?
I am searching for a multicopter blade that can provide a thrust of 15 - 20N.
Being someone who doesn't have any experience in avionics, I searched online for such blade believing that I could find the thrust the blade can provide, but every website for a multicopter wing, only the diameter is provided.
Is there a formula that provides thrust of a blade from its diameter?

The thrust depends on other parameters than the diameter: speed of rotation and pitch angle being the most obvious.

I don't have any experience in Avionics, but the project I am doing, requires a rotor/fan/blade that can provide that thrust, I tried searching for a multi-copter blade, but I can only see the diameter of the blade in the specifications

You may try to look at questions like this one: What is the highest possible thrust generated by 15cm ducted fan?

Not according to the pulling disk theory, that is what causes the confusion.

I checked out your original post, and the Quora post referenced in it. The thing is that what is mentioned there is simple impulse dynamics: there is a magic disk with no drag that accelerates air going through it. It's a theory that works if you realise the limitations, allows for quick ROM estimations, and can provide even more useful info when some drag effects are included in the form of propeller/rotor efficiency. The Quora post is not incorrect, but it is a simplification and it is valid for a different scale.
For a magic disk, all you need to know is the disk area, it does not even need to have a mounting axis. For a real propeller/fan that is not optimised yet, the following parameters need to be considered:

Blade radius, resulting in the disk area.
Rotor RPM, together with the blade radius resulting in tip velocity.
Blade pitch
Blade chord
Blade profile
Number of blades

Between your application and the Quora helicopter disk, there are the following differences that need to be catered for:

Your rotor is small. Reynolds effects start playing a big role here, with a much thicker boundary layer. Departs further from the impulse theory than an average helicopter rotor: increases power demand.
Your rotor is ducted: reduces power demand by about 35% for small rotors, see figure below.
Your rotor is a fan: increases power demand. Every added blade requires proportionally more power for a an increase in thrust that gets smaller with every blade.
Your rotor has difficulty bringing the air in. A curved duct restricts incoming air flow, increasing power demand and putting a cap on maximum thrust.

So we need to use data from small scale rotor research. Here is a masters thesis researching just that, unfortunately with an underpowered motor so the results are only valid for low Mach numbers. If we look at the highest Mach number in the graph and compare data points for Thrust = Lift, we can see an increase in $C_T$ (is a measure for thrust) of 0.047/0.035 = 35%.

$C_T$ is defined here with reference to the ducted fan wetted area, and it is not very clear which units are used. There is a graph of thrust force vs. rotor rpm:

6000 rpm with a 9.5" rotor equates to a tip Mach of 0.22. Propellers can run with a tip speed of up to 0.75 Mach before compressibility effects occur, that would be 20,000 rpm for this configuration. Extrapolating the curve to 20,000 rpm would result in about the 15 - 20 N mentioned in the OP.
It's a 3-blade rotor. Thrust can be further increased by increasing blade pitch and increasing number of blades, each increase requires an increase in power as well. Unfortunately the research does not mention power required for each thrust config found, but:

Appendix A has some provisional power curves for other rotors used in the project. It also mentions the use of a gearbox to increase rotor rpm, something OP may wish to consider as well.
A table with a motor trade study used in earlier research is included, outlined below.


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Why did it take so long for helicopters to be developed/in use?
The Sikorsky_H-5 was made in 1943 but used an engine released in the 1930's. It seems to me that the used that applied to helicopters, specifically in warfare, could have applied much earlier. Things like SAR, Medivac, spotting and ground support would all have been useful in the Second World War.

Where do you get 1948 from? Your own wikipedia link states it first flew in 1943, and the first production aircraft was delivered in 1945.
It also links to it's predecessor the R4 which was developed earlier and also saw some service in WW2

@Nigel Harper, whoops, my bad misread an "3" as a "8" but I think the question still applies.

Some of those roles were fulfilled using rotorcraft in WWII - See List of Rotorcraft used in WWII

@RedGrittyBrick, Thanks for that. Most of those refer to very limited and experimental use. I guess I should clarify my question to ask why there wasn't far broader use.

All of the roles you mention were fulfilled by helicopters in the Korean war which was only 5 years after the end of WWII.  Development in the late 40s and early 50s was very rapid and a common early 50s type, the Bell 47, was very capable first flying in 1946 and still in widespread use today.  By the way, the R22 uses engine technology that would be recognised in the 30s.  Tried, tested, simple easily understood and maintained and more or less bullet proof.

The useful payload and the robustness of rotorcraft were not sufficient for the rough conditions on the front. Only when turbines were available with their much better power-to-weight ratio and high reliability did helicopters offer a real advantage to the military.
The de Cierva autogyro line was begun in 1920, but de Cierva focused on building unstallable GA aircraft, not weapons. The same can be said for Henrich Focke, who improved on de Cierva's designs (which he built under license) with the Fa-61, which is considered the first practical helicopter.
The only helicopters to be used by the military were the designs of Anton Flettner who used two intermeshing rotors with considerable success, the transport helicopter Fa-223 with two rotors on outriggers and the Sikorsky R-4. The Flettner 282 and the Fa-223 were ordered by the Luftwaffe, but development took a long time, with many new problems to be overcome. In the end, they came too late to be used in any quantity.
The Sikorsky design was unstable and needed considerable pilot attention. Just watch how poor Igor is working the stick to keep the VS-300 straight! It also needed years of refinement before it could be used by the military. This was the R-4 which saw limited use for rescue missions from 1944 on.
Just because something can be demonstrated once does not imply it is ready for widespread use.

This is all true, I would point out that Sikorsky helicopters did see active service in the pacific theater during WWII, mostly search and rescue work.

@GdD: You are right, how could I forget the R-4! Edited.

Mr. Sikorsky's hat fits him quite well!


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Off-center fan in geared turbofan - does this exist?
Has anyone ever heard of a geared turbofan in which the axis of rotation of the fan is offset from (i.e. not the same as) the axis of rotation of the turbine? In all the current geared turbofans I've encountered, the input shaft from the turbine to the gearbox is co-axial with the output shaft to the fan.

This seems like it would cause issues, because unlike a turboprop, the core of a turbofan is very dependent on the fan flow and compression.

great, thanks for the help!

There are no off-centered turbofan engines in service today. However, United Technologies Corporation (UTC), has patented an engine with an angled core (Pratt & Whitney is a part of UTC).

Image from US patent Gas turbine engine with separate core and propulsion unit
US 8789354 B2
Air from the fan is ducted around the side and back of the core to enter from the opposite direction. The hot gas will be discharged forward through a power (low-pressure) turbine connected to the fan via a gear system. The turbine, gearbox and fan will be connected through a short shaft.

Source: aviationweek.com
The design is expected to have some advantages due the unconventional layout adopted, like:

As the core and fan are separate, core can be easily removed for maintenance.
As the compressors (in cores) are at an angle to each other, the possibility that uncontained failure in one engine will damage another engine is reduced (when the engines are installed in the double bubble D8 configuration)

great information, thanks!

In case you're interested, US Patent 7,752,834 has what I was looking for.

Interesting. I guess one benefit is smaller fan diameter.

Did you have an application in mind / what sparked your interest? Just curious.

authors: Данил Задолбался


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Are cargo holds pressurised these days?
This question is only regarding aircraft that seat more than about 100 people.
Are cargo holds pressurised these days, what is the situation?
Are only some pressurized, most, of every single one? Does it depend on the carrier, operating region or are there other variations that matter?
Related questions:

Are cargo holds heated?
Do they have lights? (Aren't some lit for animals?)
I've noticed random mentions of "some" of the sections being pressurised. Is this correct? It would seem to me that, if indeed, only some are pressurised then, of course, you absolutely could not rely on your luggage being in a pressurised area.
and what about Fedex-type cargo-only aircraft?

To be clear, I ask this question re "today" because I noticed when googling on this, there is a lot of information, but only old information (say, 10 yrs old plus). That is a recipe for confusion and urban myth, so the total facts from you experts would be great.
(Possibly, it would be difficult to secure accurate online references for this - my quick searching anyway only revealed patchy, out-of-date looking stuffs. Note for example, the Wikipedia article on Cabin Pressurization has only one poor, no-referenced sentence on the whole matter!)
Note for example... Spray bottles (pressurized) in the checked luggage?

@Falster Welcome to Aviation SE! Of course, politeness is encouraged in both answering and asking questions, but just the occasional "thanks" to a good answer is sufficient :)

Lol good one shorts -- hey guys, I made this question, but I only then linked-through my usual SO account - but now I can't "tick" it unfortunately. If someone can sort of tick it on my behalf, go for it, cheers!

(Heh I was just able to "vote up my own question!" whoo! :) )

I seemed to be able to hack it and tick the answer, thanks again gang.

Aircraft design has not changed that much in the last 10 years. In fact, most aircraft in production 10 years ago are still in production.
The cargo holds in typical airliners are indeed pressurized. Take a look at a cross section of an airliner (A380 here):

The round shape of the fuselage outline is very efficient at withstanding pressure. Because of that, everything within the fuselage shape is pressurized. This includes the cargo hold below. Only cargo holds located behind the aft pressure bulkhead would be unpressurized, and these are mainly found in smaller aircraft.
The floor of the passenger cabin is not designed to withstand that pressure, because the flat surface would need to be much heavier to do so. Decompression events are considered by regulations though, so there are vents that allow the pressure to equalize. In older aircraft without these vents, a decompression event can cause the floor of the cabin to collapse, as in this incident (and because it wasn't addressed, this one too).

Are cargo holds heated?

Sometimes. This will depend on the aircraft type. There are many threads over on the TechOps forums about this. The temperature can be adjusted depending on the cargo load.
Although the air outside the plane is cold, the walls are insulated (also for fire protection), and being pressurized along with the cabin helps too. Even in unheated cargo holds, the temperature should be above freezing. For example, the cargo bay of a 767 stays above 7˚C but with heat stays above 18˚C.

Do they have lights? (Aren't some lit for animals?)

They do have lights, mainly for ground crew during loading and unloading. See this Boeing page about fire protection where it mentions that "another safety feature on many newer airplane models prevents the cargo lights from operating in flight".

and what about Fedex-type cargo-only aircraft?

These aircraft are still based on the airliner models (and sometimes converted from airliners). Therefore, they have the same pressurization system. The cabin is still pressurized, or they would have to put in some type of bulkhead behind the cockpit.
This includes the cargo version of the 747 (both upper and lower areas are pressurized).
The rare exception would be aircraft like the 747 LCF (Dreamlifter). The cargo section is not pressurized, and there is a bulkhead behind the flight deck section.

I thought the accidents you link to were the reason why now the cabin floor must be able to withstand the pressure difference between a pressurized cabin and a vented cargo hold. FAR 25.365(e) requires any structure to withstand sudden depressurization loads.

@PeterKämpf Is the requirement to withstand those loads indefinitely or just very briefly until the passenger cabin also vents? I thought the DC-10's solution to the floor collapse problem was mostly just to install vents in the floors so that if the cargo hold depressurized, the passenger cabin would depressurize before the floor blew out.

@reirab that is what I was trying to address with my recent edit.

Logically speaking, all cargo holds must be pressurized unless the aircraft flies below 10,000 feet or the type of cargo is restricted to no liquids, and no pressurized liquids like aftershaves, shampoos, medicines etc.
At higher altitudes the atmospheric pressure drops and if the cargo hold was unpressurized, all such bottles would burst causing possible damage to the cargo and/or aircraft.

It has much more to do with structural integrity of the aircraft than with structural integrity of the passengers' shampoo bottles!

Ah yes. I got to see the results of a un-pressurized load in a DC-6 in Alaska that had a bunch of potatoe chips burst at 5000-7000AGL. They don't ordinarily do that but I guess it was a bad production run.

A change of 50-70 kilopascals will not cause an aerosol can to burst. Most shampoo and aerosol messes are caused by poor packing, for aerosols this may be the cover/lid pops off and the button is depressed by contact with other items in the luggage during handling. For shampoo it is excess airspace in the bottle causing the problem, if you squeeze out all air then there will be no significant volume change with the pressure. Vapor pressure["boiling point"] of H2O at 25c is 3.2KPa, this is the pressure at 24 000m(79,000 feet) airliners have an operational ceiling of about 13 000M/43,000ft.

Yes, they are pressurized and climate controlled, especially on commercial flights where live cargo eg pets, bees, chicks, etc may be loaded aboard in the spaces and would be lethal to the creatures to be in a space at subzero temperatures and without supplemental oxygen encountered during cruise at high altitudes.


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Does a secondary surveillance radar need the support of a primary surveillance radar?
ATC technologies are evolving to include other components than primary and secondary surveillance radars, but I believe these two pieces of equipment are still the backbone of ATC. Primary radar (PSR, 2.8 GHz) works usually on higher frequencies than secondary radar (SSR, 1.030 GHz).

Top: M10SR SSR | Bottom: Morava PSR | Right: Sky Search-3000 PSR+SSR co-mounted
Primary radar uses the natural echo reflected by the target to determine its angular position and range, while secondary radar uses the cooperative answer provided by the target transponder. The answer is equivalent to a natural echo, but is delayed by a known time, and is more powerful. It can also contain additional information such as level.
I read that SSR is more accurate in position determination than PSR. Indeed SSR doesn't work without the support of the airborne transponder, and the target can disappear from SSR display when the transponder has failed or has been disconnected.
My questions:

In appearance SSR + transponder can provide information to ATC without the need for a primary radar support. Is it true, if not why?
SSR can technically receive a natural echo on 1.030 GHz before the SSR reply which is sent on 1.090 GHz, is this signal used?
Do ATC systems usually display PSR data for controllers continuously or only on demand (e.g. when a target transponder doesn't answer SSR interrogator)?

SSR can work independently and this is quite common as well. Primary radars cost a lot to maintain, and as more and more planes are equipped with transponders, primary radars are being shut down. Many areas now have secondary radar coverage but no primary radar coverage.
SSR only relies on replies sent as a response to its own interrogations. This works well in theory, but becomes a problem in practise in areas where many SSR units transmit on the same frequency resulting in many overlapping replies. This results in FRUIT and garbling.
For areas where both primary and secondary radar data is used, the data is combined to create individual radar plots that are shown to the controller. The controller typically does not have to worry about the exact source of data, and will not worry about switching on or off individual sources (at least not during normal operations). Modern ATC units receive radar data from many different sources, including both SSR and PSR stations, and process and combine this data to a single representation. This happens by using technology such as ARTAS. On the controller radar screen, aircraft plots will look different depending on their data sources. For example, a PSR-only target may be shown as a circle, an SSR-only target as a diamond and a SSR+PSR target as a square. The exact specifications vary from system to system.

So, nowadays, in a lot of areas, an airplane whose transponder fails will become invisible to radar entirely, and ATC will have no way of keeping other airplanes from colliding with it... yeah, eliminating PSR sounds like such a great idea.

PSR is a separate system to SSR. The accuracy of SSR means that some ATC systems are switching off their PSR except for operation in the terminal area. And for those only limiting them to what is flying within a certain radius of the radar head.
The reason for this is mainly cost. The cost of running a PSR is huge. The reason though for keeping them on in the terminal area is the advantage of a aircraft without a working transponder can still be seen and separated from others.
The natural echo of an SSR system is not used. What is sent back to the SSR head is a data package. In that package is all the information about the aircraft's identification (4-digit octal code) and altitude. Some SSR transponders provide a lot more detail(aka auto-pilot settings and position, mode-S) and some older ones just identification code. The echo, if used, would only give a position that would take up more processing power process with no additional benefits to the ATC system. The return of the code is what is used for position calculations.
All surveillance data is displayed continuously. The radar tracks may be joined together to create a combined symbol or changed to a completely new symbol. Some systems have the ability to degrade the system resources at a local level but no controller I know would ever deliberately do that.
In my area of operations we get aircraft that we see them on PSR, SSR, ADS-B, and ADS-C all at the same time. We never turn any of them off we just use them all.
Sorry about my original answer I put some Mode-S information in a way that assumed it was all PSR equipment, what I said was incorrect and has been corrected. Thanks J. Hougaard for picking up my mistake.

I think you are confusing SSR with mode S. You claim that an SSR reply contains information about altitude and position, but this is not necessarily true. The most simple form of SSR is mode A, which only transmits a 4 digit transponder code. The more common one, mode C, transmits two things: transponder code and altitude. The position is still calculated by the ground unit, just like a PSR.

@J.Hougaard Thanks, I have done a basic correction. I will re read my technical manuals next time before I answer a question on information that I don't use daily. Lesson Learned

I've finally discovered exactly how SSR/MSSR calculates position, and I'm still utterly confused how this is not clearly stated all over the internet and in actual teaching material:
The trick lies in that the SSR/MSSR beam azimuth is very narrow, only a few degrees. The system calculates a transponder's distance from the radar through timing the interrogation-response-time, and since it does this 30ish times (MSSR uses far less pulses) every time the beam hits the aircraft it will be quite accurate. It now knows that the aircraft is at a given distance from the radar inside a corridor that is very narrow, i.e. it knows its position, just as you can figure out your position from a VOR/DME using a radial and distance.
The beam is a cone of course, so the further from a radar you are the larger the margin of error. Still, a MSSR can be as accurate as 1-2nm at a distance of 60nm. Adding in data from several radar sites allows systems to triangulate and provide even better accuracy on the situation display.

What unit in nm?  nano-meter?

Nautical miles (The radar sends EM waves is in the 2-30cm wavelength region, nanometers only come into the picture at the induvidual pulses back and forth from the radar)

"The trick lies in that the SSR/MSSR beam azimuth is very narrow": Azimuth isn't determined by the direction of the signal but derived from the wavefront direction by interferometry. The antenna is divided in two halves to receive the reply from two close but separate locations, phases are compared to sense the wavefront plane. This way azimuth resolution can be 0.7° with a typical 2 or 3° beam width at 3dB (wider beam ↔ increased range for the same rpm).


text:
What level of information does a PIC have about a missile crossing their aircraft route?
Recently North Korea launched a missile out of its national airspace on a trajectory which crossed the route of an aircraft from Air France. The aircraft wasn't in danger at any time, the minimal distance between the missile and the aircraft was always higher than 100 km (7 minutes separation).

Source: CBS
ATC was not able to inform the aircraft, or any other aircraft in the vicinity. If the timing was such the missile could hit the aircraft it seems this would have made no difference.
From a pilot in command, responsible for on-board safety, stand point, what kind of information is available regarding such events? Is there some protection possible yet, or expected in the near future? If not, can (or must) a PIC asks for something to be put in place?

While it's not really disseminated to civilians (pilots, ATC, or otherwise,) there are actually lots of currently-deployed sensors tracking missile launches around the world. The United States and its allies as well as Russia and China all have advanced missile-tracking capabilities and have had them for decades, dating back to the initial development of ICBMs. The U.S. Missile Defense Agency has a host of radar and infrared/visible light sensors that are used to detect launches and track missiles, for example.

There are passive missile launch detection systems available (with or without automated reaction to IR-missiles), but I doubt these systems are available for civilian markets (with perhaps the exception of Israel). Active systems can warn you if you are locked on and a launch has taken place (radar guided missiles). This will not warn you in case an ICBM will cross your path, because you are not the target. Commercial airliners will not know it is coming fed by info from the onboard systems alone.

Pre-announced missile launches will often be published by responsible countries in NOTAMs (Notices to Airmen), which are notices of potential hazards to aircraft in flight or on the ground. These are generally things like closed runways, inoperative radio navigation aids, cranes on approach or departure paths and the like. They can also warn pilots about conflict zones, missile/rocket launches, and satellite re-entry paths.
NOTAMs are published publicly and are read by pilots as part of flight planning. I've heard of important ones being read off by ATC, but that's extremely rare. Commercial flights can also be sent warnings using the ACARS system by their corporate support groups.
The NOTAM system relies on organizations/governments posting these notices. North Korea does not announce missile launches before they happen, or even after they happen (unless they are successful or course), so nobody in civil aviation knew about it until it was all over.

The part about NOTAMs being issued by responsible countries and responsible countries not including DPRK are true. However, the "nobody knew about it until it was all over" part probably isn't true. I'd suspect these guys knew about it within seconds of the first stage booster firing. Same for their Russian, Chinese, etc. counterparts.

@reirab I've amended it to say nobody in civil aviation knew about it in order to satisfy the pedants ;)

I'd love to see that set of notices: "Signage on Taxiway L not painted; Runway light out on runway 27; gopher holes on Taxyway C; nuclear missile test crossing your airspace today; coffee maker broken in lounge..."

We have NOTAMs, ASHTAMS, BIRDTAMS, SNOWTAMS, maybe we need OHS**TTAMS.


text:
What altitude to fly on a STAR when it reads "expect"?
I just took the instrument knowledge test and one of the questions referred to this STAR. Like many of the questions, there were two right answers and you were supposed to pick the one that was most right.
(airnav)
The question stated that you were cleared for the STELA ONE arrival from the west and asked what altitude you should be at when crossing STELA. My understanding of a clearance for a STAR is that, unless otherwise instructed, you should use both the lateral and vertical guidance on the chart. So at STELA, you should be at 11000'. The notation for expect isn’t applicable unless you were told to fly at a higher altitude when given your clearance. Even then, you wouldn’t descend to 11000' until instructed.
Is my understanding correct, or am I missing something?

The exact wording of the question / clearance would be very helpful in order to give an accurate answer...

"Expect" is just that......"expect". Usually the "expected" happens, but not always. My understanding is they are for planning purpose only. The actual altitude you fly would depend on any clearance or instructions received.

@Lnafziger I think I quoted the question exactly. I don‘t remember the exact wording of the answers. I can’t find the question in the latest Gleim book and it is not in my database of previous questions, so I don’t know what the exact wording of the answers was. Two of the answers said to cross STELA ONE at 11,000. I think the wrong one said to descend to 11000 at STELA ONE while the correct one said to follow the charted procedure.

Terrible question (not your question, but the test question).
As far as I can tell, there are NO altitudes on the arrival that are mandatory; everything is an "expect" altitude only.
When you are "cleared for the STELA ONE arrival", you are cleared to (required to) fly the lateral ground track, and comply with any airspeeds stated on the chart ("cross at xxx knots", not "expect to cross at xxx knots" restrictions).  That clearance by itself doesn't clear you for any altitudes, even ones that are mandatory on the chart.
When you are cleared to "Descend Via the STELA ONE" arrival, then you are additionally cleared for all mandatory altitudes.  (Which I don't see any of on this chart, but I may have missed one -- NOS formatting isn't the best.)  Importantly, if the arrival has published altitudes to cross A at or above FL 230, cross B at or below FL 200, cross C at 12,000', and expect to cross D at 5000' (and no other altitudes after D), then you are ONLY cleared down to 12,000' -- you are NOT, in that case, cleared for the 5000' altitude.  You can expect it (i.e. load it in the FMC), but you can't depart 12,000' without explicit ATC clearance (i.e. don't spin  the 5000' into the altitude window on the mode control panel until ATC says "cross D at 5000").
Yeah, it's a "gotcha".  Really poor human factors design for an arrival to do that, IMHO, but there are a few of those out there.
Now, flying the STELA ONE, you're PROBABLY going to end up crossing STELA at 11,000', but until they assign you that altitude, you aren't cleared to do so.  "Expect" altitudes on an arrival have to be explicitly given by ATC -- "Descend Via" doesn't include those.
I don't know what other answers you had on the knowledge test besides 11,000', but my first guess would be that something along the lines of "not enough information given to know" or "cannot be determined" or "last assigned" would be better than "11,000" if the question asked something along the lines of, "having been assigned this arrival, at what altitude are you cleared to cross the STELA waypoint?"  With only clearance for the arrival itself, you haven't been cleared for ANY altitude change.
I don't much like that arrival since it has no mandatory altitudes at all, and I really don't like that question.  Have to love the FAA, sometimes!

"descend via" isn't really part of this question and kind of clouds / confuses the issue here....

@Lnafziger  It seems relevant in that in the absence of Descent Via, you aren't cleared for the published altitudes, even the mandatory ones.  As you point out, such a clearance wouldn't be appropriate here anyway.  But being cleared for the STELA1 arrival, you're 0 for 2 in terms of ways that you could cross STELA at 11,000... you aren't cleared to descent via, and an expect isn't a clearance anyway.

All I was suggesting is that since there was no mention of "Descend Via", explaining how it would work isn't necessary and could confuse the issue. Especially since you refer to "when given a descend via on the STELLA ONE arrival" which isn't something that should happen....  Removing that would make it more clear. Anyway, just a suggestion!

If you're given a Descend Via, then you use the STAR for vertical guidance, and usually those clearances are given on RNAV STARs. Otherwise, you'll be given explicit descent instructions the Expect is just a planning tool, such that you can be prepared if you're given, for example a Descend Pilots Discretion, cross STELLA at 11,000, or some other variation and plan ahead for your descents.

The exact wording of the arrival clearance is critical in order to properly answer your question.
Since this arrival only contains "expect" altitude restrictions, the clearance should also contain a vertical clearance (controllers are not supposed to give "descend via" clearances for arrivals which only have "expect" altitude restrictions).
If no vertical clearance was given along with the arrival, then you must comply with the previously issued altitude clearance, and you may not descend to comply with the altitudes published on the chart until you have been specifically cleared to do so.  I believe that is the whole point of the question.
In this situation, I would still keep track of what is needed in order to make the published restriction, and if it starts getting close query ATC to see if they want me to cross it.  This just helps to keep me from getting "slam dunked" and needing an excessive descent rate if they "forgot" about me for a few minutes.

I haven’t flown any STARs, and can’t find anything in the FAA docs about how to descend, but on the NBAA website they say "Climb via” and “descend via” are abbreviated ATC clearances that require compliance with the procedure's lateral path, associated speed restrictions, and altitude restrictions published on the SID or STAR. " https://www.nbaa.org/ops/cns/pbn/climb-via/ Your answer seems to conflict with the NBAA guidance. Are you saying that in the real world, the published altitudes are not followed and that you will always be given an altitude when flying the STAR?

@JScarry  No, Lnafziger & the NBAA are consistent here.  IF they cleared you to descend via (and not just cleared the arrival), then you'd be cleared for the mandatory altitudes, but not the "expect" altitudes.  They will usually assign you those altitudes that the STAR tells you to expect, but until they do, you aren't cleared for them.

@jscarry Absolutely. However, there was no mention of "Descend Via" in the question.....


text:
How far back from the final approach fix can you intercept the glideslope?
I was looking for the point that you engage your auto pilot approach glide slope before the final approach fix in this question. How far back can you go?

It depends on the airport. ATC is required to give you the glideslope intercept altitude for your specific approach when they issue your approach clearance, and that is the altitude that you should maintain until you get established on the glideslope.

@jwzumwalt One could argue that you should maintain that altitude, but you aren't necessarily required to. When ATC says "maintain [altitude] until established," you are established when you are on a lateral segment of the approach. Once established, you can descend to the lowest published altitude of the segment and intercept the glideslope at that altitude if you wish. (AIM 5-4-6 e 1)

Depends upon the approach.  In general for the ILS, the glideslope intercept will be at the approximate position of the outer marker or 4-7 miles from the threshold.


text:
Why the pilot in command sits on the left?

Is it a rule?
Why left?
I mean why the pilot in common sits on the left?
What is the superiority of the left side?

Possible duplicate of Why does an airplane captain sit in the left seat?

In helicopters the PiC is often on the right side.

@Arian are you talking about helicopters or airplanes?

@Farhan airplane

Airplane question has already been asked making yours a duplicate.


text:
What would be the potential thrust of an "electric jet" like Lilium's?

Lilium claims to fly with "electric jets". Throwing out the debate over whether a non-combustion compression of a stream of air is a jet or only a ducted fan, what is the potential thrust each of these could be getting?
Edit: since it's not compressing, what potential lift could the Lilium prototype engines provide, and are there examples of electric engines today that provide compression in a significant way?
Ignoring the stability issue of Lilium as well. Just curious about the ducted fan design they have. Also ignore the problems with it being electric and battery powered please :)

Full marks for the marketing guys at Lilium. What a great scheme, to call it an electric jet instead of a garden variety household fan. I reckon the range of the Lilium is only limited by the length of the extension cord.

There is no way that design creates any kind of meaningful compression, ignoring the fact that the rapid expansion by burning fuel is the main reason for compressing the air in the first place...

Related, if not duplicate: Could an electric engine provide the same performance as jet engines on current aircraft?

That's what I was thinking. Are there examples of electric engines that manage compression in any meaningful way? Also, what thrust could these then provide? The full size prototype can at least take off.

Please edit your question instead of asking in comments.

Also, not a duplicate mainly because I'm curious what thrust these specific engines could currently be providing.

Compression makes sense at high speed because the blades doesn't work well at high speed, so you need to slow down the flow in the intake, maintain speed but increase pressure along the fan stages, and decompress and accelerate at the nozzle. This process has no use at low speed.

An engine doesn't have to burn fuel to be a jet; for instance, nuclear-powered turbojets don't burn any fuel (at least, not the regular kind), but are turbojets nonetheless (or would be, if any were currently operational).

They look like 15 cm fans, thrust of which is competently ROM-ed in this answer.: about 40 to 60 Newton.
Note that the $C_T$ in the equation depends on fan solidity, the higher the number of blades and  the wider they are, the more thrust they produce, but also the higher power required. For a given fan power (and battery life) it makes more sense to increase fan diameter and decrease fan solidity.

The linked answer assumes 27,000 RPM and Mach 0.65 tip speed. That is already pushing it. +1, also for the comment under the question.

Lilium's 15 cm thrusters remain THE big mystery in the eVTOL community. Nonetheless, Lilium was successful in attracting another 'few' tens of millions of VC recently. There's a reason why helicopters use wide-diameter rotors. Has to do with the ability to lift, torque and energy efficiency of course. Interesting comparison:

This doesn't answer any of the questions asked.

Still, it shows two things: That Lilium's claims are highly dubious and that their investors are extremely gullible.

The helicopter comparison is perfect.  The Lilium may gain some efficiency by operating as a "blown wing".  There was some research done on "channel wings" years ago (this would be a "straightened channel wing").

There is a difference between a fan and a compressor: the one provides thrust, the other compression. In a turbofan engine, there are separate sets of blades to do each job.
The Lilium's ducted fan is an example of a wider class of ducted propulsor. They all work the same way, to accelerate air in one direction so that the reaction force pushes the fan or propeller in the other.
If the blade disc were used to provide compression, the device would not be a propulsor but a compressor and would not exit to the free air stream.
Blade aerodynamics being equal, the maximum potential thrust is determined by intake diameter, which is obviously rather small: a single device would power a 3 m (10 ft) span model plane quite nicely.
Stacking up crazy numbers of small fans obviously increases total thrust in proportion, but it is less efficient than a couple of big ones. Integration with the airframe aerodynamics, especially the wing, is critical. It was studied intensively about half a century ago, such as for the Vought ADAM V/STOL jetliner project (illustrated article here). One can at least say that the Lilium configuration is not optimal (there was a question about exactly that a couple of months ago, if anybody can find it).

The main difference between blower fan and compressor is the amount of blockage in the exhaust. If there is a turbine in the way, compression occurs. If the nozzle is suitably shaped and free of obstructions, air gets just accelerated.

Indeed. However the blade design parameters change accordingly and in the wider context beyond aero engines, the distinction is well established (although the term "blower" is imprecise as it is also often used for a high-throughput, low-pressure compressor).


text:
Is there a commonly recognized placeholder airline name?
I'm wondering if there is a common placeholder name for an airline, either implying a legally disclosed or fictional entity, which is frequently used?
For instance, I've seen the terms containing 'widgets' used very often in an economics context. John Doe is often used in legal cases.
*"Placeholder names are words that can refer to objects or people whose names are temporarily forgotten, irrelevant, or unknown in the context in which they are being discussed." -Wikipedia

I'm going to ask my sister Alice to ask her friend Bob about this, but she has been charged with mopery due to financial irregularities surrounding her purchase of Blackacre in Outerstan, so I'll need a few cartons of Morley cigarettes from BigCo to bribe the Lord High Executioner.

@RobertColumbia Alice can't talk to Bob. Carol is listening.

Here's a little list but I'm not really convinced. I don't think there is that one airline name that immediately pops into one's mind when thinking about a placeholder. Like "ACME" for a company, "John/Jane Doe" for a person, "foo/bar" for an arbitrary string (in programming), or "42" for an arbitrary number. But "Oceanic" seems popular albeit I've never heard of it before.

@Jamiec Eve is listening.

@DavidRicherby Eve's dropping.

The thing about "widgets" is we know almost nothing about what they are. Any kind of airline is already in a much more specific industry than a widget manufacturer. I suppose you don't want a name of an airline from a particular work of fiction, that is, you want something analogous to John Doe and not John McClane.

@PerlDuck 'Oceanic' was the name of the airline in Lost. Remember not to get on flight 815.

@DavidRicherby I just looked again, and you're right it is Eve. You obviously got the pun/joke, I've always seen Carol (or Catherine) as the third party, but it really is more commonly referred to as Eve.

@Jamiec It depends what the third party's doing. If they're eavesdropping, they're usually called Eve; if it's a consensual 3-way communication, then a name beginning with C would make sense.

Another analogy is that example.com is a domain name that was specifically created to be used as an example in documentation. Hence, its use is safe in the help feature of this website.

http://fictional-airlines.wikia.com/wiki/Fictional_Airlines_Wikia

@Jamiec: Here's the full-cast of the exciting Cryptography Games, if you're interested: https://wikipedia.org/wiki/Alice_and_Bob#Cast_of_characters

Sandpiper Air from Wings ? :-)

"Fly-EZ: The airline that can't afford to crash."

@Jamiec Mallory is inserting himself into the communications (man-in-the-middle). Alice, Bob, Carol and Dave are traditionally the parties considered privy to the communications, whereas Eve and Mallory are not privy but try anyway using various means. Sometimes they try to break things, sometimes they just want to listen.

If you want to attempt to be humorous you could use High Flyer Airways or Mile High Airlines.

I think Oceanic Airlines is well known fictive airline. It is used widely in Lost and also Alias, Castle, JAG, Category 6: Day of Destruction and in Executive Decision (the material from this movie was reused in a lot of low budget movies).

You do not want to fly on that airline!

But it ensures a weird adventure!

I don't think I had ever heard of this one.

Probably you should stop flying yourself and watch the onboard-entertainment :p

As long as Tom Hanks isn't your pilot, you'll probably be fine.

@GdD Why not? The posters say it has a perfect safety record? :-)

I would suggest Acme.
At least within certain communities, Acme is a generally recognized placeholder name for airlines. This includes Acme Airlines, Acme Red, Acme Giant, Acme Express, etc. This convention is often used in the podcast world as a means of referring to one's employing company without directly naming the company but still differentiating between many of the real airlines. However, these names may have outgrown their placeholder status in some cases and may now be synonymous with the names of the real airlines.

Artwork from the APG community

That's a really nice logo for A Company Moving Everything. Looks cute.

I remember the old Warner Brother's cartoon's used to use ACME for just about every company you could imagine.  I'd have to think one of them was an airline at some point or another.

@JayCarr Considering that (IIRC) Wile E. Coyote was able to procure rockets from Acme Corp, I wouldn't be the least bit surprised if they are into airlines as well.

Credit for this Acme logo should go to The Airline Pilot Guy, Capt. Jeff and his crew. http://airlinepilotguy.com Great podcast, by the way.

@thetoolman Agreed, though it wasn't created by Jeff Nielsen but someone from the community (I forget who).

Why not use a variation on the "generic company names" from
the aforementioned Wikipedia page?
For example, Acme Airlines, Ace Airlines, XYZ Air.
If it's a small airline, Mom and Pop Air.
Note that in the example "XYZ Widget Company," the idea is to obscure
not only the identity of the company, but also to obscure what they do.
Since you have stated you want an airline, the "widget" part of the
company name is redundant. That's why I suggested simply replacing
"Widget Company" with "Airlines" or "Air".
If the generic airline is not associated with a particular market,
an alternative is to use a placeholder geographical name:
Utopia Airlines or Air Ruritania for a national-scale airline,
Podunk Airlines for a local operation.

The small scale operation will be named Mom and Pop Air but will have Utopia livery on their aircraft and do business as Utopia Connection...

@MichaelHampton Yes! :-)


text:
What's the difference between orientation and position?
In INS (Inertial navigation system) sensors, is there a distinction between the following?

Orientation (heading, pitch and roll)
Position (latitude, longitude and altitude)

Is the orientation relative to a specific axis while the position is absolute? If yes, what is the axis?

Do you know what heading, pitch and roll are, and what latitude, longitude and altitude are...?

Related: What is the exact meaning of “attitude”, does it include translational movement?

Yes there is a distinction, pitch, roll and heading are angles [unit = radian] and latitude, longitude and altitude are linear distances [unit = meter].  Together, these six parameters define the six Degrees of Freedom that define the momentary state of a rigid object with reference to the earth.
All six are defined relative to earth axes. For the linear dimensions this is clear, we just need to define a suitable zero point. For the angles:

Pitch is the nose up/down angle relative to the gravity field of the earth.
Roll is the wingtip up/down angle relative to the gravity field of the earth.
Heading is the angle relative to a suitable earth reference, usually magnetic north or geographical north.

Conversion of aircraft axes into earth axes is a non-trivial matter and the cause of much confusion, for instance in questions comparing lift to weight: lift is always coupled to aircraft axes, weight to earth axes. The axes sets are defined as follows:
Aircraft axes

Origin: Centre of Gravity of aircraft
X-axis: in the symmetry plane of the aircraft, + = forward to the aircraft nose
Y-axis: in the span wise direction of the aircraft, + = to the right wingtip
Z-axis: perpendicular to the XOY-plane, + = down into the floor.

Earth axes

Origin: Centre of Gravity at the start of the response
X-axis: In the horizontal plane (relative to gravity), + = pointing north.
Y-axis: Perpendicular to XOZ-plane, + = pointing east.
Z-axis: Pointing to the centre of the earth = +

Edit
Yes navigation lat/lon is given in [degrees] - the INS sensor ultimately converts displacement into proper navigational definitions of course. But for measuring 3-D aircraft inertial effects the SI system is used, and displacement is internally treated as a linear motion, units [metres].

Yes the aircraft pitches, rolls and yaws - but relative to what?

Down is positive Z?  That seems backwards.

I hate to be pedantic, but latitude and longitude, are angles not linear distances...just angles relative to the centre of the earth though the Greenwich Meridian.

@MontyHarder It is counterintuitive for aeronauticers, but it is defined this way to comply with the corkscrew rule: twist a corkscrew from X to Y-axis and you find positive Z

@Trevor yes indeed, lon and lat are angles. There are a couple of assumptions made so that they can be treated as linear distances, following the curvature of the earth.

@Koyovis Hmm... I wonder if this convention varies by area. I don't recall positive z ever pointing towards the center of Earth in any of the math, physics, or engineering classes I took in the U.S. Granted, I didn't take any aerospace engineering classes. +z pointing to the origin in a spherical coordinate system doesn't make much sense. Literally all coordinates (except the origin itself) would have negative z, which is kind of silly. The corkscrew reasoning seems weird, since that's an equally-arbitrary convention.

@reirab Any sign convention will work, one will just need to remember to carry over appropriate minus signs. In the US, people are more used to carrying over constants etc due to the non- metric system so that is where the difference may be. If I remember correctly, the corkscrew definition is a requirement when Euler transforms are done - which is what you do when transforming from aircraft to earth axes. You are free to choose 2 of the three axes, sign of the 3rd one is then determined. And yes I have made a few errors with this counter-intuitive definition as well, it doesn't look right.

@Koyovis By "an equal arbitrary convention," I meant that you can just as easily design a corkscrew that turns either way. Which way a corkscrew turns is just an arbitrary convention, just as which way an engine turns is an arbitrary convention. Granted, with a corkscrew a decent argument could be made that designing it one way is better for most people (the right-handed ones,) but either way is certainly possible.

@reirab it most certainly is possible to define it any way you desire, as long as you state the sign conventions etc in the docs. One can also have the x-axis point in any other convenient way than north.

Position is where you are; orientation is which way you're pointing (including any roll).

Changing the word "pointing" to "facing" includes roll (in my mind at least)

I've added "including roll" because I agree that's not included in "pointing" and I don't think it's implied by "facing", either.

If you twist your hand while pointing, you have roll covered as well.

"position" is absolute. You can think of it as the x, y, z coordinates of the aircraft in the 3D space.
"orientation" is the state of the aircraft relative to its own axes:

Image from https://en.wikipedia.org/wiki/Aircraft_principal_axes
"position" (coordinates) - https://en.wikipedia.org/wiki/Geographic_coordinate_system
orientation -
https://en.wikipedia.org/wiki/Aircraft_principal_axes

Sorry, but position is never absolute, and orientation is not a state, but a set of angles.

@bogl An object's orientation in a specific point of time definitely describes its state.

A state is a complete description of a system. The position, orientation, speed vector, and angular speed vector together describe the state of a rigid body. Orientation alone is far away from completely describing the state of a plane.

Also it's not really accurate to say orientation is the state of the aircraft relative to its own axes: relative to its own axes, it doesn't move at all!

@psmears It doesn't move at all relative to its axes? This is totally wrong.

@DeepSpace: No, it's 100% correct, because when the plane moves (pitches, rolls, yaws), its axes move too.

@psmears Don't be so picky.  https://en.wikipedia.org/wiki/Moving_frame

@DeepSpace: I'm not being picky - sorry if it seems that way - but this is an important point. You can express change in the aircraft's orientation relative to its axes, but you can't usefully specify its current orientation that way - you need to relate it to some other, external reference. To clarify: can you give an example of how would you give the current orientation of an aircraft relative to its own axes?


text:
Are there any "alternative" passenger airline business models?
(By passenger airline in this question I mean scheduled carriers, with at minimum a few aircraft with 50+ seats.)
I am aware of 3 main classes of airlines:

Hub-and-spoke (most major airlines)
Low-cost carriers (Southwest, Ryanair)
Regional (contractors that support more "spokes" for the majors)

Are there more? If so, where and how are they used?

Note: I accepted the answer that shows a clear example, but there are many other excellent answers below!

There's also the "Snowbird" model, like Sun Country and Allegiant Air, which primarily fly vacationers and snowbirds between sunbelt (San Diego to Florida) and the Carribean, and the Northern snowbelt states (Maine through Washington State, and Canadian border cities). They are low-cost carriers, with few or no code shares, no connections, no free amenities, and prices that decline as the date gets closer.
Here is Sun Country's route map.  Note that nearly all their flights are between Minneapolis (or Dallas) and vacation destinations.

Here is a part of Allegiant's route map, showing cities with flights to Phoenix/Mesa, AZ.  They have more routes than are shown here, but all their routes have one end in one of the "sun belt" cities marked in orange.

What are "snowbirds"?

People who live in the north in the summer and in the south in the winter.

4. Business class airline
Airline which operate aircraft only have Business class for medium/long haul route.
Example: La Compagnie (Paris Gaulle / London Luton - Newark)
Some airline also operated All-Business class service in some of their route.
Exapmle: BA Club World London City (London City - JFK), and cancelled Singapore SQ21/22 (Newark - Singapore).
Check Wikipedia for further detail.
Note: Wikipedia claimed that Lufthansa also have All-business class flight but i cannot find such a seat arrangement in seatguru.
PS: SAS have all business-class services between Stavanger and Houston (SK951/952) with b737 operated by Privatair. Qatar also provides all such services between Doha and Heathrow (QR 15/16) with A319 but the service will be terminated soon. (Thanks for @Calchas )

Yes LH 408/409 on DUS-EWR; LH 412/413 on MUC-EWR; and LH 484/485 on FRA-EWR used to be all-J back in 2007. Operated in a 44J configuration I think (not sure about that). Flights operated by PrivatAir. Ceased a while ago alas.

QR also do DOH-LHR all J as QR 15/16 IIRC. However, none of these planes have more than fifty seats.

Also, the old Midwest Express was only one class. I only flew with them once, but man do I miss their fresh cookies...

Even within the different categories in the question, there are multiple business models, which can be quite different from each other.
"Low-Cost Carrier" Category

Within low-cost carriers, you have Southwest on one end of the spectrum, which is kind of debatable as to whether it should even be considered a low-cost carrier these days. They charge almost no extra fees for anything and offer frequent flights to almost all major cities, but have base fares that are usually quite comparable to those of the major airlines on the same routes. JetBlue also follows a similar model to Southwest.

On the other end of the spectrum, you have airlines like Allegiant or Wow Air which charge extra fees for just about everything except cabin pressurization, mostly just fly into small-market airports, have very limited route options, offer only a few flights per week on a given route, and often don't offer connections. These are quite different business models.

In the middle of these extremes, you have the business models of airlines like Spirit and Frontier, which have fees and schedules in between those of the airlines on the ends of the spectrum.

Regional Category

Within the regional category, one business model that exists in the U.S. is that of the airlines that exist primarily to provide Essential Air Service (EAS). EAS is a government-subsidized program designed to ensure that scheduled passenger service is available to remote areas that are not near cities large enough to support significant scheduled passenger service profitably on their own. Airlines operating these routes still usually ferry passengers to larger airports where they can connect to other airlines like other regional carriers, but often operate much smaller aircraft (e.g. 13-20-seat aircraft vs. the 50 or 70 seat aircraft common on normal regional carriers.) Airlines exist almost solely to run these types of routes include Great Lakes Airlines and SeaPort Airlines (which ironically has its largest hub in Memphis, which isn't a seaport and doesn't offer service to any seaports.) For examples of the kinds of routes these airlines operate, here's the route map for SeaPort:

Traditional regional carriers within the U.S. typically operate 50-70-seat regional jets. These carriers are usually contracted to 'do business as' the regional wing of one of the major airlines (e.g. Delta Connection, American Eagle, or United Express.) They usually fly from hubs of the airlines they contract with to small-market airports, but not as small as those served by the EAS program. Additionally, the regional carriers are used to provide service between an airline's smaller hubs and medium-market or even large-market airports. For example, in the case of Delta, while a city may have mainline Delta 757 service to Atlanta, it may have only regional airline service to their smaller hubs like MSP, CVG, and SLC.

"Low cost" is surely a description of the airlines' cost base, not a description of the fares it charges—at least from an industry analyst perspective. You will always find airlines charging a lot of money when they can.

+1 just for charge extra fees for just about everything except cabin pressurization

@Calchas True, but Southwest's cost base isn't all that much less than the majors these days; that was my point on that part. It's somewhat so, but not nearly to the point of the airlines on the other end of the LCC spectrum. It's gradually been moving closer and closer to a normal major carrier over the years. It's already pretty close to a hub-and-spoke model, even if it's not officially one. MDW, PHX, DEN, LAS, BMI, BNA, etc. all operate lots of connections for SWA. Of course, they don't have the long-haul flights or interline agreements that the majors have, though.

Just saw that I typed BMI in the previous comment. I meant BWI (Baltimore/Washington,) not BMI (Bloomington, IL.) Southwest does not have a focus city in Bloomington, IL, as far as I know. - haha

Yes. The three categories there are not really reflective of many aviation businesses around the globe.
Formerly full service, not yet LCC
In many parts of the world (especially Europe) you have older carriers desperate to reposition themselves as low cost carriers. They have inherited large cost bases and they are trying to shed them, often with limited success. They often adopt the tactics of a "low cost" airline such as charging for amenities and operating on point-to-point routes. But their cost base (per available seat mile) remains as high as even the legacy, global carriers.
So they look like an LCC but actually they would not be considered an LCC by an investor. (Indeed many of them remain owned by sovereign funds.)
In the UK, Flybe (BE) might be a good example, although it is not in government ownership. BE charges low(ish) fares, it focuses on point-to-point routes, most of its routes are under 500 miles, it charges for snacks, it charges for luggage and there is no business class. However, until very recently it had a typical cost per available seat kilometre exceeding that of British Airways shorthaul!

LCCs are Flybe's biggest competitors and their lower average unit cost across their networks allows them to under-price Flybe when they do compete on the same city pairs. Even without direct competition, LCCs have a downward impact on pricing in many markets as they compete for discretionary travel spend. This could become a major threat to Flybe, especially if the level of overlap with LCCs grows.

http://centreforaviation.com/analysis/flybe-a-niche-occupied-by-few-other-airlines-but-still-with-a-unit-cost-disadvantage-200156
On the flip side BE offers feed to airlines such as KLM, Cathay Pacific and British Airways. It also wetleased aircraft to Finnair in a joint venture with them called Flybe Nordic (an affiliate member of oneworld). These are not activities typical of a low cost carrier.
I think I would also put CityJet (WX) in this category. It was recently demerged from the AF/KLM group. WX does have a "base" at London City Airport (LCY), but it really focuses on point-to-point rather than transfer traffic (LCY is too expensive to compete for transit traffic). Costs and fares are high, but it's a good airline.

This weak unit revenue trend, at an airport [London City] that is considered to be high yield, is troubling, especially given the capacity cuts. Although the relative trend of RASK [revenue per available seat-kilometre] versus CASK [cost per available seat-kilometre] started to move in the right direction in 2013, it will be a considerable achievement to restore breakeven and then to convert that into sustained profitability.

http://centreforaviation.com/analysis/cityjets-new-owner-intro-aviation-to-acquire-new-fleet-rebrand-move-ho-to-dublin-peter-oncken-176467
LCCs now offering full service amenities ("hybrid" carriers)
Many of the original low cost carriers are close to saturating the "low cost" market in Europe and realize the only way to continue growing is to offer fares and service levels that are acceptable to business passengers—or indeed, discerning leisure passengers.
EasyJet (U2) has a new CEO who is making a strong push for business traffic by using central airports, better times, reliability, offering fast lane access, a loyalty programme called "EasyJet plus", and so on. Revenues have grown strongly.
http://centreforaviation.com/analysis/easyjet--ryanair-squaring-up-to-each-other-as-head-to-head-competition-increases-208098
A direct competitor to Flybe on UK domestic routes, whom I use quite often, is a relatively new airline called Eastern Airways (T3). T3 goes the opposite route: luggage is free, all of its passengers get fast track at all airports, it provides a free lounge at its hub, there is free coffee/beer/wine/snacks on board. Yet it charges low fares (most of the time), offers no feed or interline connections, does point-to-point only, and it only operates a fleet of turboprops (and one SAAB).
(However most of their planes have fewer than fifty seats, so maybe this example isn't directly relevant.)

“We’re offering a frequent business schedule with an all-inclusive on board service, both of which are equally important to business travellers.”

http://www.anna.aero/2014/01/22/eastern-airways-returns-onto-leeds-bradford-southampton-route/
All premium long haul carriers
Other posters have mentioned the annoyingly named La Compagnie (try asking "Pardon, où est l'enregistrement pour la Compagnie?" in CDG...) offering all-busines service on PAR-NYC and LON-NYC. There have been one or two of these airlines in the past but they always struggle without the benefit of a major airline to rescue their passengers when the plane goes technical. Odyssey Airlines wants to try the same thing on LCY-JFK, but presently no aircraft can do the route without a stop.
http://centreforaviation.com/analysis/all-business-class-north-atlantic-airlines-will-odyssey-airlines-epic-tale-end-in-triumph-171835
However all of these operations are relatively "niche" and are only operating with about 30-50 seats.
Low cost long haul
Some think this will be the future. The obvious examples are Norwegian (DY) which operates a number of all economy services between Europe and USA and a few between Europe and the far east. I think they deserve to be in a separate group to low cost short haul. The challenges of operating a long haul service are quite different to low cost short haul; on many routes offering only economy and economy plus products is going to be difficult to sustain without sophisticated revenue management work to milk every cent out of those passengers.
Eventually, these operations will require feed from other low cost carriers, they'll require local "hubs" or interchange points, and that's when things will get interesting.
AirAsia X is one example of an airline already going down that route. It has a strong relationship with the various AirAsia brands and offers full interlining between shorthaul and longhaul services. Indeed, the low cost terminal at KUL had to be redesigned to enable bag-collect and re-drop prior to customs/passport control to enable better flow.
Conclusion.
I would agree that many of the airlines I've cited here to rebut your three categories are a little bit exceptional, but I think AirAsia X and EasyJet are the airlines to watch because they are not constrained by starting in one category. The three "niches" you gave will not be hard dividing lines for long.

What do all-J and all-Y refer to?

I do apologise @FreeMan, means all economy or all business. Will fix.

No sweat, just have to remember that not everyone here is a pilot/airline professional. It's helpful to explain terms like that the first time you use them in every post - you never know where someone new will start reading.

Yet another airline model is local carriers that do not primarily act as spokes, but only for their own niche markets. I believe the planned California Pacific Airlines intends to pursue this model.
Arguably, charter airlines can also be considered another airline business model. Rather than operating on a fixed schedule, these companies will fly on demand. I believe these types of airlines are often used by travel agencies in Europe to ferry vacationers to their destinations. For instance, Germany's Hapag-Lloyd/TUIfly, as well as Germany's former LTU, works this way at least for some of their business.
There are similar small-scale charter operations flying to Southern California's Santa Catalina island, as well as into the Alaska wilderness (and likely also in many other places, such as Yellowstone, Montana)
Another airline business model is specializing on sightseeing tours - flights that depart and land in the same place. LTU used to offer such flights to the North Pole. On a smaller scale, many similar companies exist in or near scenic locations, such as Alaska, Hawaii, the Grand Canyon, and probably also many international locations (Kenya's Serengeti, for instance).
Yet another business model is the private jet shared ownership.

5. "Travel-agent" airline
"Travel-agent" airline stands for airlines owned and operated by the travel agents. It often refers as charter airline, but in this case the lessee and lessor are same company.
The business model of "Travel-agent" airline looks like LCC. They usually provides homogeneous seats; their ticket are only available through their companies; their routes mainly focus on short/medium distance, but with some long tourist cities (like Bangkok) in busy seasons (Summer holiday/Christmas).
On the other hand, they often provide free checked luggage allowance and sometimes free meal. Their seats are generally wider and larger; personal entertainment system may be installed for each passengers.
Thomas Cook airlines and Thomson Airways are tow largest "Travel-agent" airline in UK.

please do not post multiple answers to the same question. you already have an answer here, if you think that it is incomplete, edit it and add this information in there.

@Federico Because they are 2 difference answer, and the first answered has been written long time ago

the age of the answer is not relevant. and no, they are not 2 different answers: both says "yes, they exist", you simply list 2 different kinds, but the answer to the question is the same

see also the "scenario 2" in this meta post: http://meta.aviation.stackexchange.com/questions/1641/1467

@Federico These answers come under scenario 1. They're 2 completely different answers, so it's fine.

@DannyBeckett I'm in chat


text:
How can ADS-B be secure when it is based on self-reporting?
By all reports, ADS-B is the future of air navigation and ATC. In the simplest definition, ADS-B is aircraft self-reporting GPS position to benefit everyone.
How is this secure? Seems like someone could simply broadcast a false position for any number of nefarious purposes.
Maybe I am missing something obvious, but I have yet to see an explanation of ADS-B from a security perspective.

Related: https://security.stackexchange.com/q/34313

Related: http://tech.slashdot.org/story/12/08/22/2059207/faa-denies-vulnerabilities-in-new-air-traffic-control-system

You are spot on! I watched a Defcon talk video a few weeks about this very topic, you might find it interesting! https://www.youtube.com/watch?v=CXv1j3GbgLk

How would any of those nefarious purposes be enabled better by sending false position reports, than by sending no position reports at all?

Right, not sending a position report at all would be a false position.

@digitgopher In which case what's different from the current system, where no one sends position reports?

what do you mean by secure? be sure this is the device that send positions (with some authentification procedire), be sure the device is behaving as it should (with some redundant checks that can easily be overridden such as the transmission time)? It seems you exclude the deny of service (i.e. a device that is not operational, whatever the causes)

An comprehensive survey of the security of ADS-B has been done by Strohmeier et al, http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6940209&url=http%3A%2F%2Fieeexplore.ieee.org%2Fstamp%2Fstamp.jsp%3Ftp%3D%26arnumber%3D6940209

Move along citizens, nothing to see here.

You aren't missing anything obvious, ADS-B spoofing (the sending of faked messages) and tampering (putting incorrect information into a message) is possible as there is no message authentication built into the protocol. The receiving station has no way to check whether messages are fakes. There are some proposed solutions here and here, to my knowledge there has been no decision to change the protocol standard.
Keep in mind that even if you add message authentication to the protocol there would still be ways to send faked messages. ADS-B uses GPS, there's no reason you couldn't fake the GPS data being sent to a broadcast unit. The unit would have no way of knowing the GPS data is not genuine, and would send position information thinking it was authentic. So even with authentication and security in the broadcast protocol you still cannot implicitly trust the information sent as potential attackers have access to the actual transmission hardware and systems. This means in order to trust the information you have to validate it using another source.
The FAA has been close-lipped about it, they say they have the situation covered but they won't say how. It may be that they feel that the continued use of radar in built-up and secured areas (there's no plans to ditch radar completely in favor of ADS-B) will allow ATC computers to filter out the fakes. Radar is computerized, some smart algorithms would be able to match radar data to ADS-B broadcasts and alert when ADS-B data doesn't match up. An aircraft trying to fly into the Washington DC zone would not be able to use spoofing to get close to a sensitive target for example.
This would mean that the areas where spoofing is more likely to be successful would be areas where there is no radar coverage. Yes, someone could mask their aircraft's true location or fake an airplane in these locations, however the benefits of the system would seem to outweigh the potential risks.

TL;DR: "It isn't. The FAA would prefer you didn't think about that."

How many places in populated countries actually have no radar coverage, though? At very low altitudes or behind mountains or something, I would expect this, but I wouldn't expect it to be a terribly common situation. Even when flying around at 4,500 ft. over rather rural areas, I see my transponder being interrogated by at least 4 or 5 different radars.

There's a reason ADS-B was trialled in Alaska.

@GdD Fair enough.

They do have the situation covered. Spoofing—or jamming—ADS-B requires a radio transmitter with significant power and that can be tracked down and whoever installed it might have been seen or left fingerprints or DNA traces etc. So you can spoof it or jam it, but the police will come for you and put you in jail (because interfering with safety-critical system is illegal) and there is simply not that much to be gained compared to the risk.

If you care about going to jail that would be a deterrent, and most people do @janhudec. Some people don't though

@GdD, well, the people who don't can't really gain that much by disrupting it anyway.

Yes, one can send deliberately wrong position reports. But why would anyone want to do that? We're assuming malicious intent, of course -- but even so?
Could an attacker cause a collision using fake position broadcasts? It is hard to see how. The core meaning of an ADS-B broadcast is basically, "Dear ATC, please don't clear anyone to fly too near such-and-such point in space, because that's where I am." Getting an unsuspecting other aircraft to avoid a particular point in space where in reality nobody is, will not in itself cause any accident. At most some distraction and swearing and "no sir, I reallly don't see that traffic" transmissions.
An attacker might mount a denial of service attack against the airspace by flooding an air traffic controller's monitor with dummy aircraft, so he can't find any safe way to route the real planes he's talking to. But for that effect it's not really necessary to send any fake position reports -- simply jamming the 1090 MHz downlink frequency would do just as well, and not be significantly more illegal or harder to detect.
You could try to convince ATC that someone else is in a different place than they really are. However, unless you jam the entire frequency, ATC would still be able to hear their real position reports, and their computers would be able to tell that they have several inconsistent reports from the same airframe address. It would be simple to handle that by separating traffic from both reported positions (which they need to be able to anyway, just in case someone accidentally misconfigures their transponder with a wrong address), and then we're back in the DoS scenario.
Could a suicidal attacker go up in an airplane of his own and get to ram into somebody else while faking his position as far as ATC sees it? If that's your goal, it would certainly seem helpful to stop broadcasting yoru true position, but that doesn't mean that broadcasting a false position would buy you anything that you don't also get simply by pulling the plug on your transponder so you don't broadcast anything. In both cases your problem would be that you're now a primary radar echo that doesn't correspond to any transponder reply. Depending on where you pull the trick that may or may not set alarm bells ringing -- but to the extent it does, broadcasting a different position wouldn't do anything to silence them.
One thing you can do is rig your ADS-B Out to transmit a wrong altitude and hope to get clearance to pass directly above or below someone which you might then try to ram without being seen -- because primary radar is generally bad at distinguishing altitudes. However, that is not a risk specific to ADS-B -- good old SSR also depends on airborne transponders to self-report their altitude in Mode C, and that can be tampered with just as easily.

While perhaps an interesting intellectual exercise, these arguments are quite beside the point. The question isn't why would someone..., the question concerns the ramifications on the ability to secure the airspace system.

@digitgopher Secure it against what? You can't really have a valid discussion of the security ramifications of something without first understanding what you're trying to defend against.

Also, AFAICT ADS-B is only being pushed as a replacement to secondary radar, so there is nothing much that can be done with a false report that can't be done with turning off the transponder already.

I apologize for my quick judgement; I now see where Henning is coming from. At first I saw the first few paragraphs as a red herring. I think the key here is that ADS-B is not a substitute for primary radar and other security measures. +1 for this post.

(not exactly an answer, I know... a bit more of a discussion)
Okay, ADS-B is vulnerable to spoofing.  What could be done about that?
Although the FAA hasn't been exactly forthcoming with their plans, I've seen articles in the past that suggest they can match up the ADS-B report, the receiving station, and a radar return for some level of discrimination.   Doing a match-up, a ground-receiver in Montana would automatically ignore any location info that claims to be in Florida.  A plane in Montana could spoof its exact location by a few dozen miles, but couldn't lie enough to be in an entirely different ARTCC.
Because it is a digital format, one idea is to make transmitters have digital certificates, so they must sign each transmission with their certificate.  By default, all receivers would see all messages, and a spoofer could still insert false messages.   However, once a transmitter is identified as sending out bogus messages, it would be trivial to set a "Ignore all messages from Tx 1357" filter.  The problem is coming to the initial recognition that a transmitter is lying in the first place.  And once its false messages are ignored, the big question is "So... this plane isn't where it claims to be.  Where is it actually??"
In short: there are mitigation steps, but nothing that can automatically and completely secure a system when anyone can be a transmitter.

How do you make a radar think you're several dozen miles from your actual location? You can spoof your altitude to a Mode C installation easily enough, but your (lat, long) location seems like it would be pretty hard to spoof by more than maybe a few hundred feet.

@Reirab: That is the point.   You can lie with ADS-B and say you're over one location, but if Radar says you aren't there, your ADS-B message can be rejected and marked as fraudulent.  However, Primary Radar does have a significant error in its reporting due to slant-range and other factors.

To add to @BigNutz's post.
Multilateration is a fairly secure way to ascertain a transmission is accurate.
TDOA (Time Difference Of Arrival) would need at least four synchronised receivers and is pretty much the exact opposite of how GPS works (you determine your location by knowing the distance of at least four satellites).
More is better of course, but you can authenticate that someone is transmitting from those 3d coordinates. It doesn't allow for authentication of who is transmitting the ADS-B however.

So anyone questioning the level of chaos and risk associated with someone "spoofing" aircraft into an active Air Traffic System utilizing ADS-B as a part of or in whole for ATC airspace surveillance, needs to rethink the issue. Without going into the weeds, one needs to simply look at the NYC class B area on FR24 to easily see one of the most complex and congested parcels of airspace in the world. If even a single ghost aircraft is surreptitiously inserted into this system in the wrong place at the wrong time, the result could be disastrous.
While the FAA and international community naively chose to create a system with no encryption or integrated signal integrity verification, they have acknowledged that it is a vulnerability and have developed options. The most likely and prevalent is the use of integrated, real time, parallel and persistent signal multilateration (MLAT) to verify aircraft positions. The long of the short of MLAT is triangulation using Time Distance of Arrival (TDOA) capabilities. Basically if enough receivers are able to determine an aircraft's distance based on how long it takes for it's transponder signal takes to get from the aircraft to the receiver, they can determine the position.
It would be extremely difficult to create a set of signals with the complexity needed to mimic that of an actual aircraft and succeed at inserting a "ghost" aircraft into the ATS surveillance. When you consider that while it only takes 4 receivers to conduct MLAT, certified ATS ADS-B surveillance systems are far more robust and precise.
So, yeah. Coulda, shoulda, woulda encrypted the system, but all is not lost.